In recent years, small sized base stations such as home eNodeB are attracting attention. Such small sized base stations are located in households. There are base stations having access restrictions so that only specific users may use the base station. A case is considered where a cell B of a small radius formed by a small sized base station, having an access restriction, is located in a cell A having a wide rage formed by a regular base station. When a user, who is not able to connect to the cell B and is present near the cell B, attempts to perform communications with cell A, even if signals from a regular base station are received by a sufficient amount of power compared to background noise, signals of cell B become dominant interference and sufficient communication performance is not realized.
In a regular cellular system, even when a terminal moves, the connection is switched to a cell having the highest average receiving power among the surrounding cells. Accordingly, among the plural base stations from which signals are received, basically, the base station transmitting signals having the highest receiving power becomes the communication target. However, near the cell B having the above access restriction, there are cases where a low quality communication environment is formed, because interfering signals from cell B become more dominant power components than signals from cell A that is the communication target.
There is known a method referred to as Alamouti encoding as transmission diversity using plural transmitting antennas. Plural transmission symbols are encoded and transmitted from the plural antennas. There are known methods for using space block codes as resources used for the transmission, such as STBC (Space Time Block Code) using plural resources that are temporally different, and SFBC (Space Frequency Block Code) using plural frequency resources. These methods are applied in wireless communication systems such as WCDMA (Wideband-Code Division Multiple Access) and LTE (Long Term Evolution).
There is proposed a technology of improving the receiving performance when there is interference from other cells. Specifically, when components from some of the cells included in other cell interference are dominant, both the signals from other cells and signals of the self-cell are considered as antenna branches of MIMO, and are received by MIMO-MLD (Multiple Input Multiple Output-Maximum Likelihood Detection) (see, for example, Patent Document 1).
Furthermore, there is proposed a technology of relatively reducing the scale of the hardware by dividing a dividing process into plural stages. Specifically, a wireless communication device includes a receiving unit for receiving spatial multiplex signals in which different transmission signals are spatially multiplexed; a first signal dividing unit for dividing the spatial multiplex signals by performing a linear operation on the received spatial multiplex signals; and a second signal dividing unit for dividing the spatial multiplex signals that have been divided into transmission signals (see, for example, Patent Document 2).
Furthermore, there is proposed a technology for attaining reception data having good error rate properties by performing a relatively small number of operations without reducing the data transmission efficiency. Specifically, a soft decision unit makes a tentative decision to obtain modulation signals that have been divided at a division unit using an inverse matrix operation of a channel variance matrix, a signal point reduction unit uses results of the tentative decision to reduce the number of candidate signal points of the modulation signals that have been multiplexed, and the soft decision unit makes a correct decision by using the number of candidate signal points that have been reduced to obtain reception data of the modulation signals (see, for example, Patent Document 3).
Furthermore, there is proposed a wireless communication device for improving throughput of data transmission. Specifically, the wireless communication device includes plural antennas; a receiving means for receiving report signals from a communication opponent; two or more means for changing the number of data series according to two or more transmission methods among a MIMO multiplex method, a MIMO diversity method, and an adaption array antenna method, and combining these with the plural antennas; a selection means for selecting at least one of the two or more means based on the report signals; and a transmission means for reporting, to the communication opponent, the transmission method corresponding to the selected means (see, for example, Patent Document 4).
Furthermore, there is proposed a technology in which each base station transmits OFDM (Orthogonal Frequency Division Multiplexing) signals in which a pilot channel including pilot signals, a control channel including at least control information needed for communicating with a mobile station, and a traffic channel including information of user data, are multiplexed; and a mobile station receives request signals s from a self-cell base station and interference signals u from another cell base station 2, detects request signals s and interference signals u using a signal division technology in MIMO such as MLD based on channel information acquired from the pilot signals and received control information, and removes interference signals from the other cell base station and divides request signals s from the self-cell base station (see, for example, Patent Document 5).
Patent Document 1: Japanese Laid-Open Patent Publication No. 2007-300384
Patent Document 2: Japanese Laid-Open Patent Publication No. 2007-295549
Patent Document 3: Japanese Laid-Open Patent Publication No. 2008-109697
Patent Document 4: Japanese Laid-Open Patent Publication No. 2009-278660
Patent Document 5: Japanese Laid-Open Patent Publication No. 2009-100116
In the technology of improving the receiving performance when there is interference from other cells in a case where components from some of the cells included in other cell interference are dominant, by receiving, by MIMO-MLD, both the signals from other cells and signals of the self-cell by considering both of these signals as antenna branches of MIMO, when transmission diversity such as STBC is performed, and an attempt is made to perform MLD by combining the self-cell and the other cell, there arises the problems that the number of symbols to be multiplexed increases and the processing amount of MLD becomes significantly high.